Miso is a food made of soybeans and has an origin in ancient China. Ancient miso was introduced to Japan through the Korean Peninsula in the Yamato Court era and has been modified to be produced by Japanese original methods developed by Japanese with various efforts for fitting to Japanese climates, resulting in miso today. Miso is not just a seasoning but a good nutritious food and also a source of protein and fat. Further, intake of miso as miso soup can be a good way to eat vegetables, seaweed, and seafood used as ingredients in the miso soup. Miso is generally produced by adding rice and/or barley steamed and fermented with a koji mold to steamed soybean or grains such as rice and/or barley, mixing with a table salt, fermenting, and aging. Miso is known to develop its color during production and storage after production. A degree of coloring of miso is said to be varied according to starting materials, as well as conditions of aging (temperature and salt concentration) and sterilization by heating. Coloring is a complicated phenomenon involving various factors such as a temperature, a time (period), and a packaging material in storage, and a concentration of dissolved oxygen. A color tone of miso thus constantly changes in a series of processes from production to distribution, and to consumers, as well as to domestic storage. If miso cannot ensure its quality at a constant level as it has a varying color according to a time (season) when produced, the miso has a decreased commercial value. Keeping a color tone of miso is thus an important issue for miso manufacturers.
Under such a circumstance, there have been various methods proposed for preventing miso from coloring. These methods are classified broadly into four categories: (I) addition of other ingredients, (II) pretreatment of starting materials, (III) improvement in fermentation conditions, and (IV) others. Examples of a method of (I) include those of adding food additives such as alum, adding a sulfur-containing compound and a compound having an enediol structure, and adding extracts of Kihada (Phellodendron amurense), Kutinashi (Gardenia jasminoides), and/or Akane (Rubia argyi) to starting materials and fermenting a mixture (JP-B 41-4397, JP-B 37-15295, JP-A 2000-236834, JP-A 8-196230). Examples of a method of (II) include a method of processing starting soybeans with an enzyme. Examples of a method of (III) include a method of using a yeast having high temperature sensitivity. Examples of a method of (IV) include a method of using a gas-impermeable cup (JP-A 63-279761, JP-A 2000-245381, JP-A 58-175464).
In addition, there is an increasing interest in physiology of ingredients in foods. One of the materials having physiological functions is a group of flavonoids. Flavonoids are contained in vegetable foods, and known to have effects such as blood glucose level-reducing, hypotensive, lipid metabolism-improving, and allergy-inhibitory effects (JP-A 08-283154, JP-A 2001-240539, JP-A 2002-47196, JP-A 2005-225847, JP-A 2000-78955, JP-A 2000-78956, WO-A 00/15237, Biosci. Biotech. Biochem., 70(4), 933 (2006), Agric. Biol. Chem., 49(4), 909 (1985), Phytother. Res., 15, 655 (2001), J. Food Sci., 71, S633 (2006)).
There are also proposed methods for applying flavonoids to seasonings such as miso and foods (WO-A 98/18348, JP-A 2005-168458, JP-A 2004-290129, JP-A 9-187244). There are also methods of coloring miso to purple with sweet potato anthocyanins and coloring white miso to red with soy sauce and/or apple (JP-A 2001-190239, JP-A 9-313127). However, an effect of flavonoids to prevent color deepening (coloring) of miso is not known.
Although flavones having many methoxy residual groups are known to have effects of softening saltiness and controlling sweetness left thereby, flavonoids are not known to have effects of sustainability of saltiness. Flavonoids are further known to have effects of reducing sustainability of sweetness of high-intensity sweeteners, reducing green smell, astringent taste, and acid taste of vegetable drinks and herbal medicines, and reducing unfavorable tastes and flavors (JP-A 6-335362, JP-A 8-256725, JP-A 11-318379, WO-A 93/10677, U.S. Pat. Nos. 4,031,265, 4,154,862, JP-A 2004-49186).